On a recent insomniac edition of Newsnight, sandwiched between the terrifying Alastair Campbell and the altogether easier Alan Milburn, I was asked what I thought Tony Blair's principal contribution to British political history had been. I replied that he was surely the first post-war prime minister to give the impression of being glad to be living in the present day. Unlike his predecessors, he didn't seem beset with nostalgia for an imaginary all-white Britain, usually now located by its admirers in the 1950s. At this observation, the one-time leader of the opposition, Michael Howard, sprang indignantly to life, asking whether I was accusing previous Conservative leaders of having been racists. Luckily, I was given no chance to answer.

Those of us who were born immediately after the second world war now share an ambivalent attitude to our recent history. We feel, not unnaturally, that we missed the defining event of our lives. In Germany the epic television series Heimat has been able to offer a brilliant perspective, sweeping through from 1919 to the present day in an attempt to convince Germans of the subtle ways in which public events have shaped the destinies of individual citizens. But in Britain, where attitudes are much more polarised and the organisation that ought to commission such work - the BBC - has ignominiously funked out of either ambitious or contemporary drama, no parallel work has been conceived, let alone produced. If we take our ideas from the left, then most of us cling to the myth of a near-perfect Labour government of the 1940s embodying social ideals long fallen from fashion. If we are of the right, then we perpetuate the yet more ridiculous idea that Britain was going to the dogs in the 1970s until a saviour called Margaret Thatcher arrived to regenerate our native genius via a bracing programme of hectoring speeches, pointless war and mass unemployment.

Into this argument steps Andrew Marr, a clever fellow by anyone's standards, commissioned to do in documentary what the BBC no longer has the guts or vision to do in fiction. Producing the book of the series, Marr is shrewd enough to know that yet another irenic trot through what is in outline an overly familiar story may risk what he himself calls "autistic repetitiveness". Just how many more times can the fall of John Profumo be rehearsed? Whatever your age, the very words "Beatles", "Kings Road" and "Cecil King" are calculated to produce a universal lowering of the spirits. So Marr's invigorating method is to adopt what you might call the Ben Schott approach to history, piling up interesting detail, often through quotation, and hoping, quite correctly, that it will do more work than windy generalisation.

Yes, it's certainly worth discovering that Hitler had a précis of the Beveridge report with him in the Berlin bunker at the end of the war ("superior to the current German social insurance in almost all points"); that Nehru kept a picture of his old school, Harrow, on his prison wall; that Harold Macmillan described Keith Joseph as "the only boring Jew I've ever known"; and that in 1964 the outgoing Tory chancellor of the exchequer, Reginald Maudling, handed over to his Labour successor, James Callaghan, in the hall of No 11 with the words "Sorry to leave such a mess, old cock". But it's just as revealing and interesting to learn that Alec Issigonis, the designer of the Mini, was known to his exasperated colleagues as Alec Issigonyet; that the provider of mass holidays Billy Butlin carried a cut-throat razor in his top pocket; and that the flaming socialite Ann Fleming described her lover Hugh Gaitskell as having "a long inquisitive nose like the ant-eating tapir".

Curiosity being the supreme human virtue, Marr's own inquisitive nose is his finest feature. But clearly even he wouldn't have bothered to write such a long book - 630 pages with a fallible index - unless he had a few fish to fry. Some of these cook better than others. When Edward Heath, of all people, is described as "a political leader whose reputation deserves to be revisited", then you are contemplating the kind of sentiment expressed only by people who write books like this. You feel Marr's time would have been better spent in the Schottish labour of listing the old sailor's exotic beards, who, at one stage, I remember, included Olivia de Havilland. And it seems even more perverse to take such an unforgiving view of Harold Wilson when Wilson, at least, was coping more successfully than most with what emerges as the central quandary of the book: not just that Britain was fated to give away its empire and struggle for economic stability, but that it could never work out how to settle on a just or sensible relationship with its principal ally.

There is, in British history, from the end of the second world war and onwards through Suez and Vietnam, a painful, abject uneasiness about the United States which is expressed as much through our unjustified sense of superiority to what Marr calls the cargo cult of celebrity worship and narcissism as through any particular incident in politics. Wilson earned the contempt of my generation for offering moral support to the American slaughter in Vietnam. Yet he now seems like an admirable master of realpolitik for having skilfully avoided committing British troops. In this context, Blair's collusion in the invasion of Iraq hardly seems like the principled action of a liberal interventionist who imagined he could bring peace without bothering to bring order. Rather, Blair is positioned as only the latest in a long line of disastrously deluded, socially conservative prime ministers who turned out not to exercise half the influence they claimed over a partner far more powerful and far more ruthless than they dared to admit.

This is a broad-based book and a decent one, setting out to tell how, after a war that made democracy fashionable, a thin, religious and homogenous people became fat, sceptical and diverse. Because he understands a world beyond Westminster, Marr pays as much tribute to Terrence Higgins as to Denis Healey. Attlee's answer to an interviewer who asked "Is there anything else you'd care to say about the coming election?" - "No" - is unimprovable. But even better is the mention of how home secretaries used to sit in their office directly opposite a board on which were written the names of all those British prisoners coming up to be hanged. With the abolition of capital punishment, Roy Jenkins replaced the board with a fridge full of white wine. Ghastly man, but what a gift for metaphor.

· David Hare's collection of lectures, Obedience, Struggle and Revolt, is published by Faber. His new play, The Vertical Hour, will be seen in London next year